Conventional gas turbine engines include a compressor, a combustor, and a turbine. Air flows axially through the sections of the engine. As is well known in the art, air compressed in the compressor is mixed with fuel which is burned in the combustor and expanded in the turbine, thereby rotating the turbine and driving the compressor. The turbine components are subjected to a hostile environment characterized by the extremely high temperatures and pressures of the hot products of combustion that enter the turbine. In order to withstand repetitive thermal cycling in such a hot environment structural integrity and cooling of the turbine airfoils must be optimized.
Cooling schemes for airfoils have become very sophisticated in modem engines. The airfoils include intricate internal cooling passages that extend radially within the very thin airfoil. The radial passages are frequently connected by a plurality of small crossover holes to allow the flow of cooling air between the passages. Fabrication of airfoils with such small internal features necessitates a complicated multistep manufacturing process.
A problem with the current manufacturing process is that it is characterized by relatively low yields. The main reason for the low yields is that during the manufacturing process of airfoils, a ceramic core, that defines the cooling passages of the airfoil, often either breaks or fractures. There are a number of factors that contribute to such a high percentage of ceramic cores becoming damaged. First, ceramic, in general, is a brittle material. Second, the airfoils are very thin and subsequently, the cores are very thin. Finally, the small crossover holes in the airfoil result in narrow fingers in the core that are easily broken under load.
Another major factor contributing to cores being damaged during airfoil fabrication is that the fragile cores are handled repeatedly, undergoing many manufacturing processes, thereby increasing the chances for the core to break. In such processes, the core is first manually removed from a die and can be easily broken during handling. Subsequently, the core is secured within a mold and pressurized wax is injected into the mold around the core. As the pressurized wax is injected around the core, the core is subjected to shearing, bending and torsion loads that may either crack or break the core. The wax mold, with the core secured inside, is then dipped into a slurry to form layers of coating or a "shell". The wax is then melted out from the shell, forming a mold with the core secured therein. The shell, with the core secured therein, is subsequently heated. The heating process of the shell with the core results in different rates of expansion of the ceramic core and the shell. The difference in growth of the shell and the core frequently results in the core being fractured or broken since the shell generally expands at a faster rate than the core, thereby stretching the core and breaking it. The next step in the manufacturing process is injecting molten metal into the shell with the core secured therein. As the molten metal is poured into the shell, it may have non-uniform flow, causing shear, bending, and torsion loads on the core. As the molten metal solidifies, the core is then chemically removed from the airfoil. Once the core is removed, the area occupied by the core becomes the internal cavity for cooling air to pass through within the airfoil.
Fractures or breakage of the core during the manufacturing process is frequently detected only after the part is completed. Even a hairline fracture in the core developed at any stage of the manufacturing process will undermine the integrity of an airfoil and result in necessary of scrapping the finished part. One financial disadvantage of obtaining a low yield of good parts is that the effective cost of each usable airfoil is very high.
Another drawback is that the fragile nature of the ceramic cores results in producability constraints that limit more optimal cooling schemes. In many instances it may be more advantageous for the airfoil cooling and engine efficiency to have smaller crossover holes or more intricate geometric features. However, more intricate cooling passages are not practical at this time, since the current manufacturing process already yields an insufficiently small number of usable airfoils and has a high percentage of ceramic cores being damaged. More intricate cooling schemes would result in even lower manufacturing yields and even higher cost per airfoil. Thus, there is a great need to improve manufacturability of the gas turbine engine airfoils to reduce the cost of each airfoil as well as to improve cooling schemes therefor.